Cemented tungsten carbide, such as WC—Co is well known for its mechanical properties of hardness, toughness and wear resistance, making it a popular material of choice for use in such industrial applications as mining and drilling where its mechanical properties are highly desired. Because of its desired properties, cemented tungsten carbide has been the dominant material used on rotary cone rock bit surfaces exposed to wear, e.g., on cutting inserts used with rotary cone rock bits. The mechanical properties associated with cemented tungsten carbide and other cermets, especially the unique combination of hardness, toughness and wear resistance, make these materials more desirable than either metals or ceramics alone.
It is known in the art that for cemented tungsten carbide, fracture toughness is inversely proportional to hardness, and wear resistance is proportional to hardness. Accordingly, when using cemented tungsten carbide as a wear surface one must balance the demand for high wear resistance with the desire to have an acceptable degree of fracture toughness. A cemented tungsten carbide material having a high degree of wear resistance may not provide a sufficient degree of fracture resistance for drilling applications, resulting in a wear surface that is brittle and thus susceptible to gross brittle fracture. A cemented tungsten carbide material having a high degree of fracture resistance, while not being brittle and having acceptable impact resistance, may not have a suitable degree of wear resistance for drilling applications.
A known approach for addressing this issue of competing desired properties has been to use a cemented tungsten carbide substrate, and place a cemented tungsten carbide material over the substrate to provide a relatively more wear resistant surface thereon. In this approach, the cemented tungsten carbide placed on the substrate is specially formulated to provide a greater degree of wear resistance than that of the underlying substrate, and the substrate is formulated to provide a greater degree of fracture toughness than the surface layer. The cemented tungsten carbide used as the wear surface is bonded to the substrate and consolidated by the process of liquid phase sintering.
A known limitation with this approach, however, is that the interface between the substrate and the surface layer must be flat or planar. Thus, this approach is not useful for addressing the need to provide a wear surface formed from cemented tungsten carbide, having both a desired degree of wear resistance and fracture toughness, on a substrate having an irregular or nonplanar interface surface, e.g., an interface surface having a variable or constant radius of curvature.
A further known limitation with this approach is the reliance upon liquid phase sintering to bond the cemented tungsten carbide substrate and surface layer together. During the process of liquid phase sintering it is known that the ductile metal component, e.g., cobalt metal, liquefies and migrates across the boundary or interface between the substrate and surface layer. This migration is not desired because it reduces the intended differential between the two material compositions across the interface, causing the interface to become homogeneous and the related differential material properties to be minimized or eliminated. For example, during liquid phase sintering the cobalt metal constituent in the substrate can migrate into the surface layer, where less of the cobalt metal constituent is desired to provide the desired degree of wear resistance. In this instance, such migration causes an undesired reduction in the wear resistance provided by the surface layer. Thus, this phenomenon of liquid phase migration is known to limit the ability to control surface layer properties by use of a material differential approach.
Cemented tungsten carbide constructions known in the art are typically formed into the shape of a green part in sheet form that is sintered to an underlying substrate during the above-described liquid phase consolidation process. The above-described process of forming the green part and the finally-sintered product both limits the types of constructions that can be used to form the final product, e.g., constructions comprising complex microstructures or multiple layers may be outside the scope of practical manufacturing capabilities, and limits the types of products that can include the complex construction, e.g., products having an irregular shape or a nonplanar substrate surface (such as those developed by residual stress analysis), may also be outside of the scope of practical manufacturing capabilities. In many rotary cone rock bit applications, it is desired that a portion of the bit or cutting element having a nonplanar surface comprising a layer of cemented tungsten carbide disposed thereon for purposes of improving wear resistance and fracture toughness at that location.
It is, therefore, desired that functionally-engineered composite surfaces, for use with rotary cone rock bits, be prepared according to principles of this invention in a manner that does not adversely impact the physical properties of either the substrate or the surface material, e.g., in a manner that avoids ductile phase metal migration, when compared to wear resistant surfaces applied by liquid phase sintering method. It is desired that such functionally-engineered composite surfaces be formed in a manner that permits use on substrates having irregular or nonplanar interface geometries. It is further desired that functionally-engineered composite surfaces of this invention provide an improved degree of wear resistance and fracture toughness when compared to conventional cemented tungsten carbide surfaces formed using liquid phase sintering methods.